Mastitis is an inflammation of the breast that occurs most commonly in breastfeeding women; it can be related to tissue injury, infection, or both. Breastfeeding mastitis is typically caused by bacteria that enter the breast through a cracked or sore nipple.

Signs of infection may include:

• Increased pain, swelling, redness, or warmth in an area on a breast.
• Red streaks extending from a breast.
• Drainage of pus coming from a breast.
• Swollen lymph nodes in the neck or armpit.
• Flu-like symptoms, such as fever of 38.3 °C (101 °F) or higher, chills, and fatigue.

Treatment usually involves antibiotic medicines, keeping up milk flow, and measures to relieve discomfort. While they are being treated, most women can safely continue to breastfeed or pump breast milk.